发臭 (fā chòu) is a verb meaning 'to become foul-smelling' or 'to rot,' typically describing organic matter—like food, garbage, or animal carcasses—that has begun decomposing and emitting a strong, unpleasant odor. The character 发 (fā) here functions as a verb meaning 'to emit' or 'to develop,' while 臭 (chòu) means 'smelly' or 'foul odor.' Together, they form a compound verb indicating the onset or process of becoming malodorous—not merely being smelly, but actively turning bad.

This word is commonly used in everyday contexts involving spoilage or decay: forgotten leftovers in the fridge, stagnant water, unclean pet bedding, or improperly stored meat. It carries a slightly negative, informal tone and often implies neglect or poor hygiene. Unlike the adjective 臭 (chòu) alone—which can describe any strong smell—发臭 specifically signals deterioration and is rarely used for intentional or neutral scents (e.g., stinky tofu is 臭豆腐, not 发臭豆腐).